Gambierdiscus toxicus Adachi and Fukuyo, 1979
Species Overview:
Gambierdiscus toxicus is an armoured, marine, benthic dinoflagellate species. It is a toxic species that was discovered attached to the surface of brown macroalgae in the Gambier Islands, French Polynesia.
Taxonomic Description:
Species in this genus are anterio-posteriorly compressed and are observed in apical or antapical view. The epitheca and hypotheca are not noticeably different in size. A distinguishing feature is the shape and size of the apical pore complex (APC) (Faust, 1992).
Cells of Gambierdiscus toxicus are large, round to ellipsoid (Figs. 1,2,7-9). The epitheca and hypotheca are discoid shaped and approximately the same in height. The cell surface of G. toxicus is smooth with numerous deep and dense pores (Fig. 1). The thecal plates are very thick. Cells range in size from 24-60 µm in length, 42-140 µm in transdiameter, and 45-150 µm in dorso-ventral depth (Adachi and Fukuyo, 1979).
Thecal Plate Description:
The plate formula of Gambierdiscus toxicus is: Po, 3', 7'', 6c, 8s, 5''', 1p, 2'''' (Faust, 1995). The apical pore plate (Po) is oval to ellipsoidal with a characteristic fish-hook shaped apical pore (Figs. 1,3,5), the opening of which is always oriented ventrally. Apical plate 2' is subrectangular and is the largest of the three apical plates (Fig. 3) (Adachi and Fukuyo, 1979). The epitheca is slightly indented ventrally (Figs. 1,7). The hypotheca is deeply excavated (Figs. 4,8,9) (Adachi and Fukuyo, 1979, Fukuyo, 1981, Taylor, 1979).
In the hypotheca the postcingular plate 1''' is triangular; its right corner extrudes, curves inside, and contacts antapical plate 1'''' (Fig. 4). The ventral end of postcingular plate 5''' curves inward so that its lower-left corner is pointed like a beak (Fig. 4) (Adachi and Fukuyo, 1979, Fukuyo, 1981). The posterior intercalary plate (1p) is broad and pentagonal (Figs. 4,6). When the marginal zone widens during thecal growth, the 1p plate changes its shape to rhomboid (Fukuyo, 1981).
The cingulum is circular, narrow and deeply excavated, and ascends slightly (Adachi and Fukuyo, 1979, Bagnis et al., 1979, Taylor, 1979). The cingular wall consists of six plates and measures nearly 5 µm in width. It is bordered by a low, thick ridge, which is made up of the folding of plates of pre- and postcingulars (Adachi and Fukuyo, 1979).
The sulcus is short, deeply concave and pouch-like, and is oriented to the right (Adachi and Fukuyo, 1979, Bagnis et al., 1979, Taylor, 1979). Along the sulcal margin, a ridge, which continues along the edge of postcingular plate 5''' and antapical plates 1'''' and 2'''', overhangs the sulcus (Fig. 4) (Adachi and Fukuyo, 1979).
Morphology and Structure:
Cells of Gambierdiscus toxicus are photosynthetic containing yellow to golden-brown chloroplasts. The crescent-shaped nucleus is oriented ventrally (Adachi and Fukuyo, 1979).
Reproduction:
G. toxicus reproduces asexually by binary fission.
Species Comparisons:
This species resembles Heteraulacus in tabulation, but differs by its right-handed girdle torsion, large apical closing plate, and a pouch-like sulcal depression (Taylor, 1979). Gambierdiscus toxicus shares a number of characteristics with G. belizeanus. They both have the same plate formula, and have similar apical pore, cingulum, sulcus, general cell shape (lenticulate and antero-posteriorly compressed), and golden brown chloroplasts.
However, they differ in a number of distinct features. Architecturally, both species have similar epithecal plates, but differ in thecal surface morphology: G. toxicus has a smooth surface with scattered fine pores, whereas G. belizeanus has a deeply areolated surface. G. toxicus is considerably larger than G. belizeanus. And plate 1p is broad in G. toxicus, whereas it is long and narrow in G. belizeanus (Faust, 1995).